title: Sticking under wet conditions: the remarkable attachment abilities of the torrent frog, Staurois guttatus.
abstract: Tree frogs climb smooth surfaces utilising capillary forces arising from an air-fluid interface around their toe pads, whereas torrent frogs are able to climb in wet environments near waterfalls where the integrity of the meniscus is at risk. This study compares the adhesive capabilities of a torrent frog to a tree frog, investigating possible adaptations for adhesion under wet conditions. We challenged both frog species to cling to a platform which could be tilted from the horizontal to an upside-down orientation, testing the frogs on different levels of roughness and water flow. On dry, smooth surfaces, both frog species stayed attached to overhanging slopes equally well. In contrast, under both low and high flow rate conditions, the torrent frogs performed significantly better, even adhering under conditions where their toe pads were submerged in water, abolishing the meniscus that underlies capillarity. Using a transparent platform where areas of contact are illuminated, we measured the contact area of frogs during platform rotation under dry conditions. Both frog species not only used the contact area of their pads to adhere, but also large parts of their belly and thigh skin. In the tree frogs, the belly and thighs often detached on steeper slopes, whereas the torrent frogs increased the use of these areas as the slope angle increased. Probing small areas of the different skin parts with a force transducer revealed that forces declined significantly in wet conditions, with only minor differences between the frog species. The superior abilities of the torrent frogs were thus due to the large contact area they used on steep, overhanging surfaces. SEM images revealed slightly elongated cells in the periphery of the toe pads in the torrent frogs, with straightened channels in between them which could facilitate drainage of excess fluid underneath the pad.
authors.affiliation: Centre for Cell Engineering, Institute for Molecular, Cell and Systems Biology, University of Glasgow, Glasgow, Scotland, United Kingdom.
authors.forename: Thomas
authors.initials: T
authors.lastname: Endlein
authors.forename: W Jon P
authors.initials: WJ
authors.lastname: Barnes
authors.forename: Diana S
authors.initials: DS
authors.lastname: Samuel
authors.forename: Niall A
authors.initials: NA
authors.lastname: Crawford
authors.forename: Ang Bee
authors.initials: AB
authors.lastname: Biaw
authors.forename: Ulmar
authors.initials: U
authors.lastname: Grafe
mesh_terms: D000222:Adaptation, Physiological; D000268:Adhesiveness; D000818:Animals; D008855:Microscopy, Electron, Scanning; D011898:Ranidae / Q000502:physiology*; D014867:Water
chemical_list: D014867:Water